Global setup for OpenCL context in tests

refs kylelutz/compute#9

device, context, and queue are initialized statically in `context_setup.hpp`.
With this change all tests are able to complete when an NVIDIA GPU is in
exclusive compute mode.

Side effect of the change:
Time for all tests to complete reduced from 15.71 to 13.03 sec Tesla C2075.
Rely on system::default_context() to hold static context

refs kylelutz/compute#9
Merge remote-tracking branch 'ddemidov/master'